A plane light source device in the prior art has been so designed that an emitting light is bright and comfortably perceived by an observer because it was dispersed and collected by means of an optical dispersing sheet or a prism sheet.
However, it is not true that 100% of the emitted light from a back light was actually emitted to a person who observes the display surface. Particularly, a polarizer allows only one of orthogonal P and S components to pass it while inhibiting the other component by absorbing it so that about 50% of the light is lost. In order to reduce the loss resulted from the polarizer, a polarization separator and a phase converter have been used.
A light incident to a polarizer can be polarized in advance by means of a polarization separator and a phase converter so that the utilization of light is improved by polarizing the light into one which can pass the polarizer.
A plane light source device is proposed in PUPA 7-64085 which has a polarization separator provided on the light emitting surface side of a light guide, the separator comprising a prism array having a ridged surface the cross-section of which is triangular or W shaped and one or more dielectric interference films laminated on the ridged surface. In this device, the light emitted from the light guide is separated into S and P components at the interface between the prism array and the dielectric interference film, or the interface between the dielectric interference films to allow one of the components (P component) to pass the polarization separator while reflecting the other component (S component) back to the light guide after repeated full reflections. The reflected light is again dispersed by a light dispersing sheet or printed dots (light dispersing members) of the light guide into de-polarized light for re-utilization of the light. Though the light is not perfectly separated into S and P components, the device is so designed as to emit one of the components more than the other component so that the quantity of light passing through the polarizer can be increased.
It is essential to this device that the light emitted out of the light guide is incident orthogonally to the prism array so that the utilization efficiency of the light is not sufficient when the light passes the dispersing sheet placed between the light guide and the prism array because it is difficult to make the light incident orthogonal to the prism array.
PUPA 6-27420 disclosed a technique in which the incident light is separated into S and P components by means of a polarized beam splitter, passing the S component through a half wave length plate to convert it to P component, and combining it with the inherent P component by means of a condenser lens for incidence to a liquid crystal cell by means of a concave mirror. This allows the portion of the polarized light (P component in this case) effectively utilized to be increased because the S component contained in the incident light is converted to a P component for combination with the inherent P component for incidence to the liquid crystal cell.
Though this technique successfully separates the light into S and P components and converts the S component to a P component for combination with the inherent P component, it is necessary that a certain distance is maintained between the concave mirror and the condenser lens, and between the concave mirror and the liquid crystal. Further, because expensive optical components, such as a beam splitter and a condenser lens, are required, they are not suitable for use in a back light of a liquid crystal display.
Further, the prior art technique involved a problem in that the S and P components which were separated from the unpolarized light became elliptically or circularly polarized in passing through a medium by virtue of the phase difference of the medium.
To resolve the above problem, the part of the inventors of this invention disclosed in a patent application number 7-155735 an entirely novel light guide and a plane light source device. This invention is an improvement to the invention disclosed in the above cited application.
It is an object of this invention to provide a light guide having a very high light utilization efficiency and a plane light source device which uses such light guide.
It is another object of this invention to provide a light guide of a variety of forms using the principle which is same as the light guide of the above first object.
It is a still another object of this invention to provide a plane light source device having more uniform emitting intensity with reduced variation of the plane intensity distribution over the distances from the light source.
It is a further object of this invention to provide a plane light source device which enables the light from the light source to be incident to the light guide at an ideal angle of incidence.
It is a still further object of this invention to provide a liquid crystal display device having a high brightness and image quality by using the light guide or the plane light source device which meets the above objects.